Recovering heavy crude oil from an oil-bearing formation is known to be difficult. One approach is the so-called ‘Cold Heavy Oil Production with Sand’ (CHOPS). In this technique oil is deliberately extracted together with sand. CHOPS is applied for example in unconsolidated sandstones. CHOPS requires a large sand influx which is maintained during the productive life of the well. After recovery the sand and oil are separated, e.g., by settling, and the sand is disposed of. Maintaining or re-initiating sand and fluid influx is one of the challenges of CHOPS.
CHOPS reservoirs have been found at depths ranging from about 350 to 900 m. The reservoir itself may have varying degree of thickness; Reservoirs are known with thickness ranging from 3 meter to 30 m.
After a new CHOPS well is started, the initial sand influx is large, but over a period the sand influx diminishes until a steady-state is obtained. At the same time, the oil recovery increases until a maximum is reached.
During the CHOPS process channels are formed in the reservoir. These channels are known as ‘wormholes’. A wormhole may penetrate several feet into a formation. Although wormholes allow the oil to flow, they may form a connection between two wells. This negative aspect may cause water encroachment or pressure loss.
After a CHOPS well has reached its maximum, production slowly declines, until at some point the oil production becomes too low for commercial exploitation. At this point a so-called ‘workover’ may be attempted.
A workover may increase the oil-production back to higher levels, by re-initiating and maintaining the sand influx. Known workover strategies include using larger diameter perforations in the oil well, using a different spacing of the perforations, apply a different type of pumping, apply fill-in wells, etc.
It is known that diagnosing the reasons for reduced oil production in a CHOPS well is difficult due to the inaccessibility of the location. Diagnostic data is often incomplete, inaccurate, or contradictory.
Indeed, placement of in-fill wells may be counter-productive. For example, if an in-fill well intersects the wormhole network, then this may render the entire well non-productive due to loss of pressure and watering out.